Tectonic Preconditioning and the Formation of Giant Porphyry Deposits
Published:January 01, 2013
S. W. Richards, R. J. Holm, 2013. "Tectonic Preconditioning and the Formation of Giant Porphyry Deposits", Tectonics, Metallogeny, and Discovery: The North American Cordillera and Similar Accretionary Settings, M. Colpron, T. Bissig, B. G. Rusk, J. F. H. Thompson
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The formation of giant and supergiant porphyry deposits is interpreted to be genetically linked to the subduction of major transform structures on the sea floor. The ultramafic crust and lithosphere associated with oceanic transforms and fracture zones undergo high degrees of metasomatic alteration (serpentinization) aided by ongoing rupture and enhanced fluid flow relative to that experienced by adjacent, structurally homogeneous oceanic crust. When the highly serpentinized (fluid-enriched) oceanic crust and lithosphere formed at these fracture zones subducts at convergent plate boundaries, the hydrous, lithospheric-scale structural weaknesses may locally develop into vertical slab tears. The formation of vertical slab tears leads to localized mantle flow and elevated temperatures at the edges of the slab. The ensuing thermal perturbations along the slab edges enhance serpentinite breakdown reactions and aids in the liberation of aqueous fluids at approximately 600°C and 80- to 100-km depth. This pressure and temperature range is close to the wet melting curve, thereby increasing the potential for slab melting and the generation of magmas such as adakites with relatively high log fo2 >FMQ. Large oceanic transforms such as the Mocha-Valdiva fracture zone in the southeastern Pacific are likely to contain the greatest proportion of serpentinite and therefore carry fluid in the form of serpentinite group minerals into the mantle. Ultimately, the volume of fluid liberated during subduction and dehydration of the serpentinized mantle at these fractures far exceeds the volume of fluid produced at adjacent, weakly altered, and “structureless” oceanic crust. We suggest that the combination of extremely large volumes of slab-derived fluids plus the potential for slab melts to develop in these regions represents a primary control on the formation of giant porphyry deposits. These conditions are met along the eastern Pacific Rim, where more than 10 large transforms are currently being subducte. The endowment of the subducting oceanic crust with transforms and equally impressive mineral endowment contrasts markedly with the western Pacific Rim where only three to four equivalent-sized transforms are found. We propose that the variation in mineral endowment between the eastern and western Pacific and the formation of giant and supergiant porphyry deposits is linked to the formation and subduction of large oceanic fractures.
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Tectonics, Metallogeny, and Discovery: The North American Cordillera and Similar Accretionary Settings
The northern Pacific Rim—for the purposes of this contribution—comprises the Mesozoic and Cenozoic magmatic-arc and associated terranes of eastern China, Korea, Japan, the Russian Far East, Alaska, Yukon, British Columbia, the western United States, and Mexico. This ~1,800-km-long segment of the Pacific Rim is marked by a broad spectrum of metallogenic environments and mining jurisdictions, which combine to dictate where and how exploration is conducted and the overriding character of any resulting discoveries.
This summary report commences with a brief metallogenic overview of the northern Pacific Rim, with particular attention paid to the world-class Mesozoic and Cenozoic ore deposits that define the region’s premier metallogenic provinces. This is followed by a summary of the relative attractiveness of the region’s various mining jurisdictions, as recorded by recent exploration activity. The major discoveries made along the northern Pacific Rim, particularly during the past half century, are then placed in this metallogenic and regulatory context as a basis for determining the successful exploration methodologies employed. This discovery track record is then used to predict what the future of exploration in this vast and varied region may hold.
Much of the northern Pacific Rim, from eastern China and the Russian Far East in the northwest through Alaska to western parts of Canada, the United States, and Mexico in the southeast (Fig. 1), is characterized by a complex array of oceanic, accretionary prism, magmatic arc, and back-arc basin terranes and associated microcontinental blocks accreted to the North China, Siberian, Hyperborean, and North American cratons, mainly during Mesozoic times (Coney et al., 1980; Campa and Coney, 1983; Kojima, 1989; Nokleberg et al., 2005; Yakubchuk, 2009). The metallogeny of these tectonic collages is dictated by various combinations of pre-, syn-, and postaccretion ore-forming events, the last of which are generally preeminent, except in British Columbia (Nokleberg et al., 2005; Nelson and Colpron, 2007).
Although the Meso-Cenozoic metallogeny of the northwestern and northeastern Pacific quadrants displays some similarities, it is the contrasts that are most marked. The main contrasts stem from the preeminence of tin, tungsten, and antimony in eastern China, Korea, Japan, and the Russian Far East and of copper and silver in Western Canada, the conterminous United States, and Mexico. Nonetheless, both the northwestern and northeastern Pacific quadrants are exceptionally well endowed with gold and molybdenum deposits. The northeasternmost Russian Far East, Alaska, and Yukon Territory display elements of both northwestern and northeastern Pacific metallogeny (Fig. 1). These metallogenic contrasts between the northwestern and northeastern quadrants result in China being the world’s leading producer of tungsten, tin, bismuth, and antimony, mostly from its eastern Mesozoic metallogenic province.